A new study indicates how the first cells might have evolved from rocks, water and hot alkaline fluid rich in hydrogen gas spewing out of deep-sea vents, and how they might have escaped their deep sea lairs.
The scientists published their findings in the journal Cell¹. Scientists thought that the origin of life was tied to the origin of the cellular ion pumps, proteins that regulate the flow of ions across the cell’s membrane. All cells have an enzyme called ATP synthase, which uses the energy from the flow of ions across membranes to produce the energy-storage molecule ATP. This process relies on ion-pumping proteins that generate these gradients. Cells store energy by the means of proteins that make ion gradients, but it takes energy to make these proteins in the first place.
The scientists think that hydrogen-saturated alkaline water, meeting acidic oceanic water at underwater vents, would produce a natural proton gradient across thin mineral walls in the rocks that are rich in catalytic ion-sulfur minerals. This could create the right conditions for converting carbon dioxide and hydrogen into organic carbon-containing molecules, which react with each other to form the building blocks of like, like nucleotides and amino acids.
The rocks of deep-sea thermal vents contain labyrinths of these thin-walled pores, which could have been the precursors of cells, producing a proton gradient and concentrating the simply organic molecules formed, enabling them to eventually generate complex proteins and the nucleic acid RNA.
Rocky proto-cells would be initially lined with leaky organic membranes. If the cells escaped the vents and became free-living in the ocean, the membranes would have to have been sealed. But sealing membranes would cut off natural proton gradients, because while an ATP synthase would let protons into the cell, there would be nothing to pump them out.
Proteins that pump protons out of the cells would solve this problem, but there would have been no evolutionary pressure for such proteins to evolve until after the membranes were closed. In which case, they would have had to evolve a proton pumping system in almost no time, which is impossible, states researcher Nick Lane.
The scientists think that the proto-cells were able to escape this dilemma by evolving a sodium-proton antiporter, a simple proton that uses the influx of protons to pump sodium ions out of the cells. As the proto-cell membranes started closing up, they became impermeable to the large sodium ions before the smaller protons. This would have provided advantages to cells that evolved a sodium-pumping protein, while they could still rely on the vents’ natural proton gradients to generate energy. These antiporters created sodium gradients as well, and when the cell membrane closed up completely, the cells could run on the sodium gradient, and were free to leave the vent.
Reference: “The Problem with Mixing Mitochondria” by Nick Lane, 12 October 2012, Cell.